1. Field of the Invention
This invention relates to planar transformers having a combination of primary coil board, a secondary coil board and a magnetic core assembly, to switching power supplies using the planar transformer, and more particularly to cooling structures thereof
2. Description of the Prior Art
Recently, switching power supplies for electronic equipments were required to be decreased in size and thickness, thus planar transformers in which a primary coil and a secondary coil consisted of multilayer boards came to be used.
Conventional planar transformers using coils such as the multilayer boards have a structure to cool down coils indirectly by fixing a cooling fin on a magnetic core. For example, it is known a structure that is disclosed in Japanese Patent Application Laid-Open No. 7-23559.
However, increase of switching frequency in DC-DC converters used for switching power supplies accompanies with increase of eddy current losses of coils in planar transformers. There is a problem that said structure such as fixing the cooling fin on the magnetic core is less effective to cool down heat generations of coils themselves caused by eddy current losses.
Conventionally, transfer mold structures are known as methods for radiating heat generation from the primary coil and the secondary coil to ambient atmosphere. However, in case of planar transformers used for resonation type DC-DC converters in which a wide gap should be provided between primary and secondary coils, it is difficult to keep the wide gap constant when the transfer molding is made. If multilayer boards of the primary and secondary coils are integrally made, it seems to bring about many useless parts and to be adequate for cooling methods.